BILLINGS, Mont. (AP) — Federal regulators offered more details on testing requirements for oil transported by rail on Thursday and warned companies against skirting the rules after a spate of explosions caused by crude train derailments in the U.S. and Canada.

The new order from the U.S. Department of Transportation builds on a Feb. 25 declaration that the industry’s unsafe handling practices have made crude shipments an imminent hazard to public safety and the environment.

Testing already was broadly required to gauge the volatility of oil, one of hundreds of hazardous materials subject to federal oversight. But there were no standards on how frequently that testing had to be done or on what parameters companies must follow.

Transportation Secretary Anthony Foxx said in Thursday’s order that within the “reasonable, recent past” companies must have tested the flash point and boiling point of crude. Such tests help determine how likely the fuel is to ignite and dictates what type of rail car can be used for shipments.

Officials also warned companies not to re-label crude as a more generic category of flammable liquid in an attempt to get around the testing.

What constitutes “reasonable” would depend on the circumstances, according to the Transportation Department’s Pipeline and Hazardous Materials Safety Administration. For example, if a company could show that its product consistently displayed the same characteristics over time, then that would be considered as reasonable, agency spokeswoman Jeannie Shiffer said.

Regulators did not define what time period was covered by “recent past.”

There’s been intense political and public pressure to make oil trains safer since a runaway train with 72 tank cars of North Dakota oil derailed and exploded in Lac-Megantic, Quebec, near the Maine border in July. Forty-seven people were killed and 30 buildings destroyed. Oil trains also have exploded and burned in Alabama, North Dakota and New Brunswick in recent months.

Unlike most hazardous materials that come from chemical plants or other manufacturing sources, crude oil is not refined before being loaded onto trains and its properties can vary greatly among shipments.

Larry Bierlein, a Washington, D.C. attorney specializing in hazardous materials regulations and general counsel for the Association of Hazmat Shippers, says that creates a “shifting target” for how to properly handle the fuel.

But Bierlein added that regulators were setting up companies for failure with the recent orders on testing, by leaving them too vague.

“They’ve weaseled out of this by saying it’s your job to classify this (oil) properly and if you have an incident then the vast amount of time you must have violated the rules,” Bierlein said.

Failure to comply with the order would subject companies to civil penalties of $175,000 per day for each violation.